The present application relates to nuclear magnetic resonance imaging and, more particularly, to a method for obtaining magnetic resonance chemical-shift spectroscopic information which is localized to a selected discoidal sample portion, using a surface coil for at least the receiving antenna.
It is known that, because of the improved filling factor and a restricted spatial sensitivity which significantly reduces the detected Johnson noise from a sample, surface coils provide improved signal-to-noise ratios, over large volume coils, for detecting localized magnetic resonance spectra imaging signals. This capability for providing an improved signal-to-noise ratio renders surface coils a popular choice for in vivo magnetic resonance spectroscopic imaging of low sensitivity nuclei, such as .sup.31 P and .sup.13 C. However, it is also well known that a serious problem is encountered when utilizing surface coils for studying tissues lying deep beneath the surface, wherein undesirable spectral contributions result from intervening surface tissue.
It has been proposed to improve surface coil localization through the use of static magnetic field profiling gradients, as in the "topical magnetic resonance" (TMR) technique described by R. E. Gordon et al. in "Nature", 287, page 763 (1980), although this proposed solution requires applying a very high continuous power to the TMR profiling coils in body scanners. It has also been proposed to utilize depth and refocusing radio-frequency (RF) pulse sequences, by M. R. Bendall et al. in J. Magn. Reson., 53, page 365 et seq. (1983), although this solution appears to have unsatisfactory surface tissue contamination away from the surface coil axis. It has been further suggested to utilize a two-dimensional Fourier transform (2DFT) method with the RF field gradient inherent in surface coil excitation and variable RF pulse lengths to obtain a one-dimensional chemical image, by A. Haase et al. in J. Magn. Reson., 55, page 164 et seq. (1983), although the accompanying application of variable RF pulse lengths in the 2DFT method results in a loss of sensitivity, and "ringing" in the Fourier domain for a small number of values of RF pulse lengths results in loss of spatial resolution.
Accordingly, it is desirable to provide a method of obtaining nuclear magnetic resonance chemical-shift spectroscopic imaging information using surface coils, without appreciable surface tissue spectral contributions and without any accompanying sensitivity loss or relatively high continuous power requirements.